Richard Neustadt, who has died in London aged 84 of complications after a fall, was for more than 40 years the pre-eminent scholar of the American presidency. He worked in the White House under President Truman and was the friend and adviser of Presidents Kennedy, Johnson and Clinton. He taught at Harvard University and was one of the founders of the Kennedy School of Government there. He was the first director of the School's Institute of Politics. He and a group of colleagues managed the transformation of the Harvard School of Public Administration into the Kennedy School.

Neustadt's first wife, Bert (Bertha), died in the late 1970s. In 1987 he married Baroness Shirley Williams, the Liberal Democratic leader in the House of Lords. He kept a home at Wellfleet on Cape Cod in Massachusetts, but the couple lived most of the time in England.

His most important book, Presidential Power, first published in 1960, influenced Kennedy as well as a whole generation of academics, and continues to be one of the staples of courses about the presidency all over the world.

Neustadt had first worked for the federal government in the Roosevelt administration before service with the US Navy in the second world war. He then served as a special assistant in the White House under Harry Truman, so he knew what he was talking about when he stressed the limitations on the president's power. In the 1950s, a number of biographies of strong presidents and academic treatises on the institution had stressed its powers and the variety of the president's roles, for example as head of state, head of government, commander in chief, party leader and "leader of the Free World". Neustadt took a radically original view.

The president, he believed, had to grab "for just enough power to get by the next day's problems". He quoted Truman, at the end of his time in the White House, as saying about his successor, Ike Eisenhower: "He will sit there, and he will say, 'Do this! Do that!' And nothing will happen. Poor Ike - it won't be a bit like the army."

Neustadt argued that "the power of the presidency is the power to persuade". To be precise, he said, the government has three assets: the power to persuade, its professional reputation, and its public prestige. In a government like that of the United States, where powers are shared between congress, the judiciary and the executive branch headed by the president, the president must do his best to bargain with rival power centres to get what he believes to be needed.

Neustadt was a liberal, with a liberal's faith in the potential of an activist presidency like that of Franklin Roosevelt. His analysis of presidential frustration was meant as advice to John Kennedy to be an activist, and Kennedy took it as such. Neustadt wrote a plan for Kennedy's first days in office called A Tentative Check-List For The Weeks Between Election And Inaugural. It advised him to emulate Roosevelt's first "hundred days" with "a first impression of energy, direction, action and accomplishment".

While other Harvard colleagues such as Arthur Schlesinger Jr and John Kenneth Galbraith accepted jobs in the Kennedy administration, Neustadt chose to stay at Columbia, where he was professor of government (1954-64). But Kennedy asked for his advice on many matters, notably about the Skybolt affair. This was a serious crisis in Anglo-American relations in which the president felt he had been out-manoeuvred by his mentor, the British prime minister Harold Macmillan.

The British government had been relying on an American-built missile, Skybolt, to continue a supposedly independent British nuclear capability after the obsolescence of the British V-bombers. Kennedy and his Pentagon advisers, however, wanted to end the Skybolt programme, but at an emotional conference at Nassau, in the Bahamas, in December 1962, Macmillan persuaded him to replace Skybolt with the intercontinental missile, Polaris. Kennedy's wish to end the in dependent British deterrent was frustrated. He was afraid that he had presented General de Gaulle with an opportunity to veto British membership of the then European Community.

Kennedy asked Neustadt to interview participants in this affair. Neustadt's secret report to Kennedy, not fully published until 1999, is regarded as a masterpiece of subtle analysis of a complex political and strategic negotiation.

Neustadt was born in Philadelphia, but his family of Swiss origins had connections with northern California, and he attended the University of California at Berkeley, where he received his BA. He earned his MA and PhD at Harvard, and was a professor at Cornell University before moving to Columbia and then returning to Harvard, where he became the founding director of the Kennedy School's Institute of Politics from 1966 to 1971.

Few political scientists have been able to draw on a wider experience of government or a wider acquaintance among politicians, bureaucrats, journalists and academics. Presidential Power was republished, and updated, several times until it appeared in its final form as Presidential Power And The Modern Presidents: The Politics Of Leadership From Roosevelt to Reagan (1990). Drawing on his Skybolt experience, he published Alliance Politics (1970), and in 1986 he co-authored Thinking In Time, sub-titled The Uses Of History For Decision-Makers.

An energetic man with a delightful sense of humour, Neustadt could also be sharp in his judgments. He famously wrote that all incoming administrations in Washington display "arrogance, adrenalin and naiveté". He remained to the end of his life a democrat as well as a Democrat. In the last edition of his book about the presidency he admitted that "personally, I prefer presidents... more sceptical than trustful, more curious than committed, more nearly Roosevelts than Reagans". Reagan's years "did not persuade me otherwise, in spite of his appeal on other scores".

He predicted that presidents "will less and less have reason to seek solace in foreign relations from the piled-up frustrations of home affairs. Their foreign frustration will be piled high too."

In early 2002, he lectured in Edinburgh on President Bush's response to the terrorist attacks of five months earlier. He compared Tony Blair's handling of relations with Washington with Clement Attlee's over Korea as Neustadt had witnessed it from the Truman White House. When Attlee refused to send a second British brigade to Korea, Neustadt heard Admiral Sidney Souers, Truman's national security adviser, characterise Attlee as "pusillanimous". Blair's status in Washington, Neustadt said, was "almost that of a second secretary of state". He was surprised that the British public had not applauded Blair for "so brilliantly endeavouring to strengthen British influence where it counts".

He is survived by his wife, Baroness Williams, and his daughter, Elizabeth, by his first marriage. A son predeceased him in 1995.

· Richard Elliott Neustadt, government adviser and political scientist, born June 26 1919; died October 31 2003